There is known a technique of etching a target object having an organic film on a base film by plasma in a predetermined pattern while using a mask film formed on the organic film as a mask. However, in the case of etching using plasma, the mask film is damaged by the plasma and, thus, a dimension of the pattern of the mask film is changed. Accordingly, CD (Critical Dimension) of a groove formed in the organic film by using the mask film is increased. Therefore, there is known a technique of curing a mask film by plasma of H2 gas or CH3F gas (see, e.g., Japanese Patent Application Publication Nos. 2016-92102 and 2014-7281).
When the target object is processed by plasma of CH3F gas, the surface of the organic film which is not covered with the mask film is also cured. Therefore, an etching rate of the organic film is decreased by the process of curing the mask film, which may result in so-called etching stop in which the etching does not proceed.
In order to increase the etching rate, a high frequency bias power for ion attraction may be increased. However, an increase in the etching rate leads to a decrease in a selectivity between the organic film and the base film. Therefore, it is difficult to etch the organic film while suppressing damages inflicted on the base film, particularly in a target object in which a thickness of the organic film is not uniform.